01 - Whisk together flour, cocoa powder, powdered sugar, and salt in a bowl. Add cold butter and rub in with fingertips or pastry cutter until mixture resembles coarse crumbs. Stir in egg yolk and 2 tbsp cold water, mixing until dough just comes together, adding more water if needed. Press dough into a disk, wrap in plastic, and chill for 30 minutes.
02 - Preheat oven to 350°F. Roll out dough on lightly floured surface to fit a 9-inch tart pan. Press dough into pan, trim excess, and prick bottom with fork. Line with parchment and fill with pie weights or beans. Bake for 15 minutes, remove weights and parchment, and bake 5 more minutes. Let cool completely.
03 - Place chopped chocolate in a heatproof bowl. Heat cream in saucepan until just simmering, then pour over chocolate. Let sit for 2 minutes. Add butter, then gently stir until smooth and glossy.
04 - Pour ganache into cooled tart shell and smooth top. Chill for 1 hour or until ganache is set.
05 - Arrange raspberries over ganache. Optionally warm raspberry jam and brush gently over berries for glossy finish.